It is now well known that the provision of reduced density aluminum alloys for aircraft use can be effected through the inclusion of lithium as an alloying agent therein. The addition of lithium as an alloying agent, while desirably reducing alloy density, often results in a decrease in ductility and fracture toughness. Where aircraft use is contemplated, it is imperative that lithium-containing alloys have both improved fracture toughness and strength properties.
It will be appreciated that both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys", Properties Related to Fracture Toughness, ASTM STP6055, American Society for Testing and Mateials 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness.
Attaining the combination of both high strength and high fracture toughness in an aluminum-lithium alloy having density reduction in the order of 5 to 15% has been a recognized objective in this art for a number of years. Such lithium-containing alloys would be of significant utility in the aircraft and aerospace industries where low weight, high strength and high fracture toughness translate to high fuel savings and other design economies.
Prior approaches directed toward such a desirable objective have included variations in thermal, mechanical and thermomechanical processing of lithium-containing alloys. One of such areas of endeavor has been the employment of modified aging treatments and particularly in the employment of multiple aging techniques. Exemplary thereof are U.S. Pat. Nos. 4,812,178, 4,323,399, European Application 0158571Al and USSR patent 703,373.